The present invention relates generally to an image display apparatus, and more particularly to a head- or face-mounted image display apparatus that may be mounted on an observer's head or face for use.
In order to allow an individual to enjoy images on a large screen, image display apparatuses, especially head- or face-mounted image display apparatuses have now been under extensive development.
Among prior art head-mounted image display equipment, there is known an image display apparatus designed to transmit an image on a CRT or other image display device to an object surface by way of an image transmission device and throw the image on the object surface in the air by means of a toric reflecting surface (U.S. Pat. No. 4,026,641).
Another known image display apparatus is an apparatus proposed by the applicant, wherein once an image on a liquid crystal display device (LCD) or other image display device has been formed by a refraction type relay optical element, the image is guided to an observer's eyeball by way of an ocular optical element comprising a decentered concave mirror (JP-A 6-294943).
One important consideration for head-mounted image display equipment is that the overall size and weight of the equipment should be reduced, and the equipment should be mounted on the head be kept intact. The overall size of the equipment is primarily determined by the layout of the optical system involved therein.
A direct view type layout designed to magnify an image display device through a convex lens for direct viewing is unsuitable for equipment size reductions, because the amount of equipment projecting from the observer's face becomes large. To allow for a wide angle of view, it is required to use a large positive lens element and a large image display device. However, this results in increases in both the size and weight of the equipment.
To achieve an image display apparatus which allows an observer to view images for hours without causing fatigue thereto and is easily attachable to or detachable from the head, it is desired that an ocular optical element comprising a reflecting surface be located just in front of the observer's eyeballs. This layout then enables an image display device, an illumination optical element, etc. to be accommodated and arranged in a limited area in the vicinity of the observer's head, so that the amount of the apparatus projecting from the observer's face can be reduced simultaneously with reductions in the weight of the apparatus.
Another important consideration is to ensure a large angle of view. This is needed to add a sense of realism in image viewing. In particular, the three-dimensional appearance applied to a presented image is determined by the presented angle of view (ITEJ, Vol. 45, No. 12, pp. 1589-1596 (1991)).
Yet another important consideration is how an optical system capable of obtaining a wide angle of view and high resolving power is achieved.
To apply a three-dimensional and powerful appearance to the observer, it is required to ensure a presented angle of view of at least 40.degree. (.+-.20.degree.) in the horizontal direction. At the same time, it is known that the effect is saturated in the vicinity of 120.degree. (.+-.60.degree.). In other words, it is preferable that the angle of view is equal to or greater than 40.degree. and as close to 120.degree. as possible. When the aforesaid ocular optical element is a planar reflecting mirror, however, a very large image display device is needed so as to make light rays incident on the observer's eyeballs at an angle of view of greater than 40.degree.. This results in increases in the overall size and weight of the apparatus.
Referring here to a concave mirror, it produces field curvature having strong concavity along its surface. When a planar image display device is located in the focal position of the concave mirror, it is thus impossible to obtain an image that is clearly discernible to the periphery of the field of view because of a distortion of the viewing image surface thereof. A method of locating the display surface of an image display device while it is distorted, too, is known as contemplated in the prior art (U.S. Pat. No. 4,026,641). Even though the image display device is located at the front focal position of a concave mirror so that the image display device can be magnified and thrown in the air by use of the concave mirror alone, however, it is then still difficult to obtain high resolving power when the angle of view of 40.degree. or greater is provided, because of aberrations produced by the concave mirror.
When a refracting type relay optical element such as one shown in JP-A 6-294943 is used, the apparatus becomes large because the relay optical element is made up of a transmitting. In addition, decentration aberration cannot fully be corrected for by the transmitting lens element. It is thus required to locate a refracting type of decentration-correction optical element between the relay optical element and the concave mirror.